Method of making dielectric materials



Aug. 10, 1948.- F. B. HODGDON 2,446,928

METHOD OF MAKING DIELECTRIC MATERIALS Filed April 13, 1945 FIG.

IN VEN TOR.

f. B. Hooeoo/v A TTO/P/VEV Patented Aug. 10, 1948 METHOD or MAKING nmmc'rmc MATERIALS Frank B. Hodgdon, Naperville, Ill., assignor to Western Electric Company, Incorporated, New

York, N. Y., a corporation of New York Application April 13, 1943, Serial No. 482,866 2 Claims. (Cl. 117138.8)

, 1 This invention relates to dielectric materials and to a method of making the same.

Heretofore, mica and similar materials have been commonly used as the dielectric body in many types of electrical condensers. However,

high quality mica suitable for use as the dielecv tric body in electrical condensers is now dimcult to obtain and also the preparation of mica to usable form entails considerable work.

Objects of the present invention are to provide an effective and efficient dielectric material and amethod of making the same.

In accordance with one embodiment of this invention, -a polystyrene film is coated with a powdered dielectric material using as a binder for the dielectric material, cellulose acetate applied in a solvent of acetone diluted with alcohol.

Other objects and advantages of the present invention will be apparent from the following detailed description taken in conjunction with the following drawings, wherein Fig. 1 is a diagrammatic view of a coating apparatus which may be used in practicing the invention, and

Fig. 2 is a perspective view partly in section of an electrical condenser made in accordance with this invention. I

In order to provide a dielectric body which will have satisfactory electrical and physical properties, a high dielectric constant material having a suitable low loss characteristic, such as titanium dioxide or various titanates of the alkaline earths, may be coated on a suitable film and the particles of the dielectric material held together and held to the basic film by a suitable binder. Polystyrene, because of its exceptionally low loss characteristic, provides a desirable base material for such use. Since the binder forms part of the dielectric coating it consequently must also haven low loss characteristic and a relatively high' dielectric constant. Thus in coating dielectric materials on a base material, a binder for the dielectric material, such as cellulose acetate, or cellulose acetate'butyrate, dissolved in a volatile solvent such as acetone may be mixed with the dielectric materials. This provides an extremely adherent binder having good dielectric qualities. However, acetone, while it is an excellent solvent for cellulose acetate, or cellulose acetate butyrate, will attack polystyrene and softens it to such a degree that if a polystyrene film be used as a base material, it will disintegrate under slight mechanical stress.

As shown in the drawing in Fig. 1, in producing a dielectric body. a strip 5 of base material, such as a film of polystyrene may be passed through a tank 6 containing a coating compound 1, through an oven 8 and finally wound on a drum 9. The film must remain strong enough throughout the coating and drying to resist rupture as it is pulled along. Since the acetone also tends to soften the polystyrene base, as stated above, as well as to dissolve the binder material, it will be apparent that the film may be weakened to such an extent that it will rupture.-

A In accordance with the present invention, however, a film of polystyrene may be coated with dielectric material, using acetone, methyl ethyl ketone, methyl isobutyl ketone, or other similar materials as a solvent for a. synthetic binder, which may be cellulose acetate or cellulose acetate butyrate, or other similar material, without danger of rupture. It has been found that the softening action of the acetone or other solvent on the polystyrene may be substantially eliminated by dilution of the acetone while still maintaining a suflicient concentration of acetone to hold the binder material in solution. The solvent is, in short, diluted with a non-solvent for the film-so that while it will dissolve the binder material, it will not attack the base. This is accomplished by adding a diluent such as butyl alcohol or any of the aliphatic hydrocarbons, which, while it wets the polystyrene, does not attack it. The diluent must, of course. be as volatile or less volatile than the solvent since otherwise as the solvent evaporates, the diluent would evaporate at a faster rate and the solvent would eventually become sufliciently concentrated to attack the polystyrene film.

A satisfactory coating composition may be produced in accordance with the present invention by diluting 500 cc. ofacetone with 1000 cc. of butyl alcohol to provide a volatile" solvent for 50 grams of cellulose acetate butyrate. The dissolved binder may then be mixed with 950 grams of titanium dioxide. The composition so obtained may be applied to a film of polystyrene without danger of substantially weakening the maximum precentage which will not attack the polystyrene base appears to be on the order of 50%.

auaoaa Alter the film has beencoated, dried and calendered, it may be cut into sheets and assembled between sheets of foil, and a pair of terminal clamps II, as shown in Fig. 2, may be clamped to either end of the assembly. Sometimes it is then desirable to mold a casing H of a material such as a phenolic condensation product about the assembly. This is done under considerable heat and pressure. Many thermoplastic materials. and particularly polystyrene, whenheated lose their shape. For example, ordinarily ii a film of polystyrene is heated to 180? F., as'it may be when a casing is molded around it, it will shrivel and shrink almost into the form of a pellet. When a casing is molded about a condenser as sembly comprising alternate laminations of coated polystyrene film and foil, the shrinkage oi the'film may sometimes cause ruptures which will short circuit the condenser.

In accordance with the present invention, a

polystyrene film coated with a dielectric coating composition such as that hereinbei'ore described, is calendered and then heated to a temperature on the order of 350 F. to relieve the strains in the polystyrene film, the coating preventing the polystyrene from changing shape while heated. Thereafter the coated film may be heated as may be required in molding a casing around a condenser containing the film without damage thereto. The film has high tensile strength and will retain its shape during the molding operation at ordinary molding temperatures.

While this invention has been described in connection with the used of polystyrene as a base material and an acetate as a binder, it will be understood that many other materials may be used and that the method of this invention will be equally applicable thereto.

What is claimed is:

1. A method of making a condenser dielectric which comprises dissolving a binder selected from the group consisting oi! cellulose acetate and cellulose acetate butyrate in a solvent composed of 10% to 50% acetone and the remainder butyl ingcomminuted titanium dioxide with the binder composition, coating the composition on a' thin film of polystyrene, and drying the coated film.

, 2. A method of making acondenser dielectric in accordance with claim 1 in which the binder is present in the amount 0150 grams of cellulose acetate, the solvent is composed of 500 cc. of

10 acetone, 1,000 cc. of butyl alcohol, and the titanium dioxide is present in the amount of 950 grams.

FRANK B. HODGDON.

15 REFERENCES crrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS 2 Number Name Date 1,249,390 Hasburg Dec. 11, 1917 1,713,997 Clement May 21, 1929 2,085,512 I Schneider June 29, 1937 2,147,824 Webb Feb, 21, 1939 25 2,177,266 Schupp Oct. 24, 1939 2,223,833 Sander Dec. 3, 1940 2,259,746 Gofl et a1. Oct. 21, 1941 2,281,602 Ruben May 5, 1942 2,307,488 Clark Jan. 5, 1943 30 2,322,353 Fruth June 22, 1943 FOREIGN PATENTS Number Country Date 1 274,841 Great Britain Aug. 30, 1938 5 400,068 Great Britain ,Oct. 19, 1933 494,574 Great Britain Oct. 24, 1938' 504,571 Great Britain Apr. 27, 1939 OTHER REFERENCES Wilson: "Pyroxyline Enamels and Lacquers," 2nd ed., pp. 5961. 

